This invention relates in general to imaging systems and print resolution enhancement and, more particularly, to mapping of lower resolution digital data to a higher resolution for subsequent printing on a lower resolution print engine.
Electrophotographic processes for producing a permanent image on media are well known and commonly used. In general, a common process includes: (1) charging a photoreceptor such as a roller or continuous belt bearing a photoconductive material; (2) exposing the charged area to a light image to to produce an electrostatic charge on the area in the shape of the image; (3) presenting developer particles (toner) to the photoreceptor surface bearing the image so that the particles are transferred to the surface in the shape of the image; (4) transferring the particles in the shape of the image from the photoreceptor to the media; (5) fusing or fixing the particles in the shape of the image to the media; and (6) cleaning or restoring the photoreceptor for the next printing cycle. Many image forming apparatus, such as laser printers, copy machines, and facsimile machines, utilize this well known electrophotographic printing process.
In laser printers, an image is typically rasterized to form a bit pattern which is stored as a binary image bitmap for subsequent rendering to a final output image. The image bitmap is also referred to as a picture element (xe2x80x9cpixelxe2x80x9d) raster image. In the rasterizing process (i.e., forming the binary bitmap), graphic elements, such as continuous lines (line art) and text character outlines are converted to pixel patterns that approximate the source image shape. Continuous tone data, such as photographic data (both color and gray value images) are also converted to pixel patterns that approximate the source continuous tone image data. However, to effectively portray the original source image for continuous tone data, each pixel of the source image must be represented by multiple bits which define either a color or a gray level and which are subsequently converted, typically, to a binary image bitmap. Hereafter, it is to be understood that when the term xe2x80x9cgrayxe2x80x9d is used, it applies to both color and black/white images and, when applied to a color image, relates to the intensity of the color.
Conventionally, in order to represent gray level images on a bi-level (black and white) printer, the pixel data, if not already gray level, is converted into a gray level, multi-bit configuration. For example, when a multi-bit configuration of 8 bits per pixel is employed, 256 gray levels can be represented by the digital pixel values. The individual gray level pixels are converted to binary level pixels (i.e., bi-level data for subsequent rendering) through the use of a dithering process. Spatial dithering (or digital halftoning) is the converting of the multi-bit pixel values (of a source image) to fixed-size, binary, multi-pixel groupings that approximate the average gray value of the corresponding source data. This dithering process provides a halftone texture to selected areas of the image so as to provide gray value variations therein. Thus, for example, with binary pixels, a 6xc3x976 multi-pixel grouping can, in theory, simulate 36 levels of gray, and an 8xc3x978 grouping can simulate 64 levels.
The dithering process (i.e., halftoning) employs a comparison of the individual pixel values (specified by a source image intensity array) against a threshold matrix (dither matrix or device best threshold array) to control the conversion of the gray level values to appropriate patterns of bi-level data. For purposes of this discussion, a gray level value of 255 in a source image is considered to be xe2x80x9cwhitexe2x80x9d, and a gray level value of 0 is xe2x80x9cblackxe2x80x9d. The threshold matrix comprises a plurality of row-arranged gray level values which control the conversion of the gray level pixel values to bi-level pixel values which are stored in a resultant page buffer array (raster) bitmap. During the dithering process, the threshold matrix is tiled across the image pixels to enable each gray level image pixel to be compared against the correspondingly, logically-positioned gray level value of the threshold matrix. In essence, each entry in the threshold matrix is a threshold gray level value which, if exceeded by the source image gray level pixel value, causes that gray level image pixel to be converted to a xe2x80x9cwhitexe2x80x9d pixel (or, in this example, a binary logical xe2x80x9czeroxe2x80x9d for laser modulation purposes in the electrophotoconductive process). If, by contrast, the source image gray level pixel value is less than or equal to the corresponding threshold matrix gray level value, it is converted to a xe2x80x9cblackxe2x80x9d pixel (or a binary logical xe2x80x9conexe2x80x9d for laser modulation purposes).
Thus far, the discussion has focused on the differences between rasterizing text (or line art) and halftone images. However, in either case, once a raster page buffer array bitmap is generated from a source image, whether the image is text, line art, or halftone, the desired output image is created (rendered) by causing a laser to be modulated in accordance with the bit pattern stored in the image page buffer array bitmap. The modulated laser beam is scanned across a charged surface of a photosensitive drum in a succession of raster scan lines. Each scan line is divided into the pixel areas dictated by the resolution of the bitmap and the pitch of the laser scan. The modulated laser beam causes some pixel areas to be exposed to a light pulse and some not, thus causing a pattern of overlapping dots on each scan line. Where a pixel area (dot) is illuminated, the photosensitive drum is discharged, so that when it is subsequently toned, the toner adheres to the discharged areas and is repelled by the still charged areas. The toner that is adhered to the discharged areas is then transferred to paper and fixed in a known manner.
In general, the fidelity of the output image relative to the source data is directly related to the resolution of pixels (dots) in the output image. Arbitrary analog images cannot be exactly reproduced by a bitmap raster unless an infinite resolution is used. For example, as a result of the images""s pixel configuration, image edges that are either not parallel to the raster scan direction or not perpendicular to it appear stepped. This is especially noted in text and line art.
Various techniques have been developed to improve the quality of the output image of a raster bitmap. These enhancement techniques include: edge smoothing, fine line broadening, antialiasing (to smooth jagged edges), and increasing the resolution of the laser printer. These enhancing techniques typically modify the signals to the laser to produce smaller dots that are usually offset from the pixel center, or in other words, to produce gray scale dots.
Although the prior art has attempted in a variety of ways to overcome the stepped appearance of pixel image edges for text and line art, an example of one of the more widely used techniques is described in U.S. Pat. No. 4,847,641 to Tung, the disclosure of which is incorporated in full herein by reference. Tung discloses a character generator that produces a bitmap of image data and inputs that bitmap into a first-in first-out (FIFO) data buffer. A fixed subset of the buffer stored bits forms a sampling window through which a selected block of the bitmap image data may be viewed (for example, a 9xc3x979 block of pixels with the edge pixels truncated). The sampling window contains a center bit cell which changes on each shift of the image bits through the FIFO buffer. As the serialized data is shifted, the sampling window views successive bit patterns formed by pixels located at the window""s center bit cell and its surrounding neighbor bit cells. Each bit pattern formed by the center bit and its neighboring bits is compared in a matching network with prestored templates. If a match occurs, indicating that the center bit resides at an image edge and that the pixel it represents can be altered so as to improve the image""s resolution, a modulation signal is generated that causes the laser beam to alter the center pixel configuration. In general, the center pixel is made smaller than a standard unmodified bitmap pixel and is possibly moved within the confines of the pixel cell. The pixel size alteration is carried out by modulating the laser contained in the xe2x80x9claser print enginexe2x80x9d of the laser printer. One drawback of the Tung method is that the pixel alteration is accomplished only by accounting for shifting uni-dimensionally in the scan direction the modulation of the laser within the confines of the pixel cell. The system taught by Tung is now generally referred to as Resolution Enhancement Technology (RET) and enables substantially improved image resolutions to be achieved for text and line art over actual print engine resolution capability.
The limitation of Tung has been overcome by techniques described in U.S. Pat. Nos. 5,193,008 and 5,134,495, issued to Frazier et al., the disclosures of which are incorporated in full herein by reference. In general, the Frazier et al. patents employ an edge smoothing technique which also change laser pulse exposure times in accordance with template comparisons to selected pixels in an image. Importantly, their process utilizes an initial binary image that is at a level of resolution (e.g., 600 dot per inch (dpi)) that is higher (e.g., double) than that which the designate printer can output (i.e., the raster capability/resolution of the printer is only 300 dpi). A logical window is then stepped, at 600 dpi, across the entire image plane. At each step of the window, the higher resolution pixel arrangement is compared to pre-existing templates and, upon a match, causes an altered modulation of the printer""s laser beam. The modulated laser beam produces on a photoreceptor not only a gray level central pixel at the lower resolution, but also sufficiently exposes the photoreceptor about the edges of a central pixel to enable scans by adjacent scan lines to combine to create intermediate pixels between the scan lines (i.e., create intermediate pixels in the process direction) which provides an edge smoothing effect.
More specifically, pixel dots are created half-way between adjacent horizontal scan lines (the horizontal scan lines defining the raster capability of the printer) by energizing a plurality of pixels on adjacent scan lines so that the sum of the energies applied to intermediate pixel points (those points defined by overlapping adjacently exposed areas) is above a threshold levelxe2x80x94thereby enabling the intermediate pixel points to be later toned. Frazier et al. employ a xe2x80x9clook up tablexe2x80x9d based upon a template view of the source bitmap. Both of the Frazier et al. patents teach that the entire image plane is created at a higher level of resolution, with the initial image data being received at the higher level of resolution, thereby requiring a substantial memory allocation. The Frazier et al. technology has come to be known as xe2x80x9cResolution Doublingxe2x80x9d.
In short, conventional resolution enhancement techniques generally require more source digital data of a higher resolution format to achieve the enhancement desired than what the printer resolution is actually capable of. However, it is advantageous to work initially with lower resolution source data rather than higher resolution source data from a data processing perspective. Specifically, it is much less time consuming (less data to be processed) and cheaper (less hardware or memory intensive) to work with lower resolution data than higher resolution data.
Accordingly, an object of the present invention is to achieve improved resolution enhancement, in both the scan and process direction, when processing source data of a resolution that is equal to (or less than) the resolution of the destination print engine. In contrast, an object of the aforementioned co-pending application Ser. No. 08/855,253 is to improve resolution enhancement when processing lower resolution source data with a higher resolution print engine.
According to principles of the present invention in a preferred embodiment, lower resolution source data is synthesized to a higher resolution format for subsequent rendering on an output device having a same lower resolution (raster scanning) capability as the source data. Synthesis occurs by selecting and using a synthesis template that represents a configuration of a plurality of pixel data in the higher resolution format into which the lower resolution source data is to be synthesized. A working or active pixel is identified from the lower resolution source data, a synthesis template is selected (or generated), and then the synthesis template pixel data is substituted for the working pixel for rendering on the output device. The synthesized higher resolution data is subsequently rendered in a manner such that dots represented by the synthesized data are formed interstitially relative to the given lower raster/resolution capability of the output device.
According to further principles, the working pixel is identified in the lower resolution data by recognizing a configuration of a plurality of pixel data adjacent the working pixel (i.e., a working template match). The synthesis template that is generated from the working template match comprises at least a two by two cell matrix for pixel placement in the higher resolution format. Pixels in the synthesis template are cooperatively formed to provide an apparent merge of the pixel data in the higher resolution format with the adjacent pixel data of the lower resolution format data. The same synthesis template may, optionally, be used at varying levels of rendering, such as for each working pixel evaluated to be synthesized and rendered, for each page strip, or for each page of data to be rendered.
Other objects, advantages, and capabilities of the present invention will become more apparent as the description proceeds.